1. Field of the Invention
The present invention relates generally to the transfer of microwave energy and more particularly to the transfer of energy from a cyclotron maser. It relates to the coaxial coupling of microwave energy between a pair of coaxial waveguides and to the transfer of microwave energy from a coaxial waveguide through an annular window. The invention has particular application to microwaves of very high power, as for use in radar.
2. Description of the Prior Art
High power microwaves have been produced by cyclotron resonance maser devices, such as gyrotron amplifiers and oscillators wherein energy is transferred from an electron beam to an electromagnetic wave. The principles of various cyclotron resonance devices, and of gyrotrons in particular, are well known. See, for example, Flyagin, V. A., et al., "The Gyrotron," IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-25., No. 6, June 1977, pp. 514-521; Hirschfield, J. L., et al., "The Electron Cyclotron Maser--An Historical Survey," IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-25, No. 6, June 1977, pp. 522-527; Symons, Robert S., et al., "An Experimental Gyro-TWT," IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-29, No. 3, March 1981, pp. 181-184; Ganguly, A. K., et al., "Self-consistent large signal theory of the gyrotron travelling wave amplifier, "Int. J. Electronics", Vol. 53, No. 6, 1982, pp. 641-658; Ganguly, A. K., et al., "Analysis of two-cavity gyroklystron," Int. J. Electronics, Vol. 51, No. 4, 1981, pp. 503-520; and Baird, J. Mark, "Survey of Fast Wave Tube Developments," Technical Digest of International Electron Devices Meeting sponsored by IEEE, Washington, D.C., 1979, pp. 156-163.
As set forth in these references, in general the various cyclotron resonance maser devices involve an electron beam moving in an axial direction in an axial magnetic field. The electrons in the beam have a substantial motion transverse of axial and hence move generally helically along magnetic lines of flux. The electrons move through a waveguide or resonant cavity containing travelling or standing microwaves. The electrons interact with the microwaves, initiating phase bunching of the electrons. The bunched electrons radiate microwave energy, which is extracted through a window separating the encoded electron beam device from the atmosphere.
The spent electrons are then collected, generally on a hollow circular tube collector. The collector tube may serve as an output waveguide for the microwaves, and the window is ordinarily a circular dielectric disk in the end of the tube. The microwaves pass through the window with some absorption in the dielectric. See Flyagin, "The Gyrotron", supra at pp. 514-521, particularly FIG. 1, where microwave energy is extracted through an end window.
The average power available from such an electron beam device of the prior art at millimeter wavelengths is limited primarily by the heating of the vacuum window at the output of the respective device and the dissipation capabilities of the beam collector of the respective device. The vacuum window problem consists of effectively removing heat generated by dielectric losses in the window material in order to prevent excessive thermal stresses and consequent cracking of the window and the leaking of air into the electron beam device. The problem with the beam collector in prior art devices is the limitation imposed on its size, and hence dissipation capability, by the use of the beam collector as part of the output waveguide.
Both of these problems have become important with the development of the gyrotron (or cyclotron maser), the interaction structure of which is capable of generating megawatts of power at millimeter wavelengths. This capability exceeds the power handling capability of the prior window designs, which consist of a single edge cooled disk or a pair of disks having between them a flowing dielectric fluid as a coolant. Both these designs have power limits, independent of their dimensions, which are intrinsic to the thermal and dielectric properties of the available materials, and which decrease rapidly with increasing frequency. The present limit is about 200 kW at 60 GHz. The beam dissipation capability of the collectors of prior art gyrotrons is likewise a decreasing function of frequency. This is because the microwave power generated in the interaction region of the gyrotron continues to travel with the electron beam, out of the interaction cavity through a taper up to the collector diameter, the collector being essentially a hollow cylinder. In order to maintain the output power in a single mode (the one in which it was generated), the taper must follow a prescribed smooth curve, and the collector wall must maintain a high degree of circularity and straightness. With increasing frequency, the mechanical tolerances become more severe, and the required taper length increases (as the square of the frequency for a fixed diameter). The taper requirements are already a problem for 200 kW at 60 GHz.